Despite epidemiological signs warning of an emerging major epidemic, no HIV/AIDS prevention programs tailored specifically for men exist in Chile. Prior work by the investigative team has shown that Chilean men recognize their lack of information and support and their need for effective HIV prevention programs. This mixed-method study will develop and test the effectiveness of Mano a Mano para Hombres, an HIV prevention program for adult males living in a socio-economically disadvantaged environment in Chile. The specific aims are to: (1) Identify the unique HIV risks related to sexuality and substance use of adult males living in a socio-economically disadvantaged environment in Chile;(2) Identify these men's level of interest in HIV prevention and how to make the program accessible;(3) Develop the Mano a Mano para Hombres program tailored to the needs of these men, and (4) Evaluate the effectiveness of the intervention in changing knowledge, attitudes, HIV prevention behaviors, and participation in community HIV prevention. Integrated through all phases of this study are the secondary aims: (5) Strengthen the skills of the PI as an independent researcher and (6) Enhance the interdisciplinary health research capacities of Pontificia Universidad Catolica de Chile (PUC). The intervention is guided by an innovative conceptual model that integrates social-cognitive learning theory, the primary health care model of health worker-community collaboration, and the PI's own model of factors affecting Chilean men's HIV risks. Phase I uses qualitative focus groups and interviews with men and health workers to identify Chilean men's needs (Aims 1 &2), and Phase II develops and pilots the intervention by integrating the insights of Chilean men, women and health care workers with the investigative team and mentors'previous experience with existing HIV prevention interventions (Aim 2). In Phase III (Aim 3), effectiveness of the intervention will be assessed using a quasi- experimental design. The intervention group (n=250) and a control group (n=250) from a comparable community will be assessed at baseline and 3 months post intervention. A sample of 500 provides adequate power to test the hypotheses. A community advisory board will ensure community acceptability, participation, and sustainability. The intervention is low-cost, uses existing infrastructure, and includes careful process evaluation. If the intervention is effective, it can be disseminated for men throughout Chile.